English previews? Don't think there are any, aside from the minor previews offered in his comics section (which is just badly translated summaries of previewed pages). I know White Lightning publications produced a handful of English translations, but these were a series of smaller comics which contained only a handful of chapters from each book (I believe only the first two books were done). Icarus publishing now has the rights to do the remaining books, though how, and even when, this will be done is unknown, even by Mr Nishimaki (who seems to have a curious disinterest about the process).
J-List has all the original Japanese books and the White Lightning translations available, if you are interested.